W

In Eq. (KK), T=2irr2KNh is the optical thickness defined above, rv is the volume-weighted moment of the cloud droplet size distribution, which can be approximated by rell , pw is the density of liquid water, and W is the liquid water content of the cloud. Using Eq. (JJ), the term t(8R/8t) can be shown to be equal to i?(l - R) (see Problem 10).

Platnick and Twomey (1994) have applied Eq. (KK) to marine clouds off the coast of California and southern Africa, to fogs in central California, and to ship tracks. Figure 14.42 shows a typical range of susceptibilities as a function of cloud droplet size. The measured susceptibilities in these studies covered three orders of magnitude, from 5 X 10"

cm for fogs to

0.8 X 10"3 cm3 for marine clouds off south Africa and 2 X 10 ~2 cm3 for thin stratus clouds off the California coast. Similarly, Taylor and McHaffie (f994) report cloud susceptibilities in the range from 10~4 to >8 X 10~3 at various locations around the world. The highest susceptibilities were those with the smallest aerosol particle concentrations below the cloud base. As the particle concentration increased beyond ~500 cm3, the susceptibility was relatively constant at ~5 X 10~4 cm3. This means that the addition of new particles to a relatively clean air mass is far more effective than for a polluted one in terms of the effect on clouds. In short,

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